Plants belonging to the genus Glycyrrhiza that are perennial herbaceous plants in the family Fabaceae are known as important raw materials for Chinese herbal medicines and widely used around the world. Parts to be used as herbal medicine in the plants are mainly roots and stolons. It has been revealed that a main active ingredient contained in these parts is glycyrrhizin in G. uralensis, G. glabra, and G. inflata belonging to the genus Glycyrrhiza. Glycyrrhizin is a sweetener belonging to oleanane-type triterpene saponin (triterpenoid saponin). Various studies such as pharmacognostic study, pharmacological study, and breeding study on glycyrrhizin have been carried out due to its usefulness.
In order to stably and continuously supply high quality glycyrrhizin as medicine by a biological production system, establishment of optimal conditions for production, selection of a high-production strain of glycyrrhizin, breeding of high production plants of glycyrrhizin by introduction of a synthase gene, or the like, are necessary to be carried out by using a biosynthesis-related gene of glycyrrhizin or a gene expression amount of the gene as a marker. In order to do so, identification of a biosynthesis-related gene of glycyrrhizin is essential. As to a synthase gene involved in a biosynthetic pathway of glycyrrhizin, biosynthetic pathways and genes involved in the biosynthetic pathways, after β-amyrin synthase, have hardly been clarified. β-amyrin belongs to oleanane-type triterpene (triterpenoid), and is a precursor from which biosynthesis of glycyrrhizin and soyasaponin diverges in the triterpene saponin biosynthetic pathway (see FIG. 1a). To date, as a synthase gene in a pathway from β-amyrin to soyasaponin, a cytochrome P450 oxidase gene CYP93E1, which hydroxylates β-amyrin and sophoradiol at the position 24, has been isolated from soybean (see Patent Literature 1 and Non Patent Literature 1) (see FIG. 1a). Furthermore, as a synthase gene in a pathway from β-amyrin to glycyrrhizin, a cytochrome P450 oxidase gene (CYP88D6), which oxidizes carbon at the position 11 of β-amyrin, has been isolated from Glycyrrhiza by the present inventors (see Patent Literature 2) (FIG. 1a and FIG. 8). However, since the biosynthetic pathway of glycyrrhizin and a synthase gene related thereto have not sufficiently been elucidated, it has not been possible to efficiently obtain glycyrrhizin by a biological production system.